Fiber optic technology has provided a medium of ever increasing importance for the transmission of wide bandwidth data signals. Long distance telephony is now essentially all digitized, and these data signals are often multiplexed along with computer communications for transport over long routes via intermediate repeater facilities. All of the circuit switching, which routes individual components of the multiplexed signal along different paths, is done while the signals are in electrical form at the repeater facilities. But there is a need for a fiber optic switch for facilities switching, that is, the replacement of portions of the fiber optic link that may fail or for the redirection of an entire multiplexed optical signal over an alternate route.
Optical fiber switches that reposition the cleaved end of one input fiber in close proximity to the cleaved end of one of two alternative output fibers are difficult to construct because of the very small fiber cores in the single mode fibers usually used in communications systems. The switching of planar guided waves in electro-optic films can be fast, but requires the coupling of polarized light from a fiber into the planar guide, and this often results in light loss. Other types of switches have been made by expanding the fiber guided light into a parallel beam by means of a lensing device. Some operate by the repositioning of a mirror or prism to intercept input and output beams in alternative ways and are usually polarization independent. Others redirect the beams by manipulating polarization states and often make use of liquid crystal media.
One type of liquid crystal based optical switch has been constructed using a twisted nematic cell, a switching element that rotates the plane of polarization 90 degrees when in the unactivated state and 0 degrees when the cell is activated by the application of an AC electric field. Arbitrarily polarized input light must first enter a polarizing beam splitter to be separated into two orthogonal plane polarized optical components. These are passed separately at normal incidence through different sections of the twisted nematic cell where their polarizations states are controlled by the voltage applied to the cell electrodes. They are then recombined into one of two alternative outputs in a second polarizing beam splitter. In the optical bypass switch described by Soref in U.S. Pat. No. 4,478,494 a single polarizing beam splitter is used both to separate and to recombine the two orthogonal polarization components, but three additional prisms are required to direct the light within the device and to contain the liquid crystal media.
A different switching mechanism is used by Soref and McMahon in U.S. Pat. No. 4,516,837. A nematic liquid crystal between two high index glass prisms results in the total internal reflection of an obliquely incident beam, but the reflection of the transverse magnetic TM wave polarized with its electric field vector in the plane of incidence (p wave) may be inhibited by an applied field across the liquid interfacial layer, while the transverse electric TE wave polarized perpendicular to the plane of incidence (s wave) will continue to be reflected. Transparent indium tin oxide electrodes on the prisms are coated with an alignment film to orient the extraordinary liquid crystal axis in the plane of incidence when the field is off. It can be realigned parallel to an applied field because of its positive dielectric anisotropy. But since only TM polarized light (p wave) is switched from reflection to transmission by an AC voltage applied to the electrodes, arbitrarily polarized input light must first be separated into its two orthogonal TM and TE components. The latter is converted to TM by a half wave birefringent plate and both are passed separately through different sections of the nematic liquid crystal interfacial layer. Both components are thereby directed to one of several alternative output ports at which are located half wave plates and polarizing beam splitter prisms to recombine both components into individual outputs.
Soref and McMahon also showed that if the alignment films on the cell electrodes were prepared so as to orient the extraordinary axis of the nematic liquid crystal interfacial layer perpendicular to the plane of incidence both polarization states could be switched. In U.S. Pat. No. 4,278,327 they describe an optical switch that uses one region of such an interfacial layer to reflect the TM (p wave) and transmit the TE (s wave) polarization components of an arbitrarily polarized input light wave after which both are reflected back to a second region of the same liquid crystal interfacial layer. When an AC voltage is applied only to electrodes in the second region the TM (p wave) component is transmitted and the TE (s wave) component is reflected, the opposite as that in the first region of the liquid crystal layer where the electrodes are left unactivated. Both polarization components are thereby directed to one of two alternative output ports by the applied AC voltage. The switch does not require separate multilayer coated polarizing beam splitting prisms.
All of the prior art liquid crystal switches described above exhibit cross-talk to some extent, and the principal cause is the partial reflection of a wave polarization component that should nominally be transmitted at some interface. In addition, the multilayer coatings in polarizing beam splitter prisms usually reflect a small but significant fraction of the TM (p wave) polarization state which should be totally transmitted. The liquid crystal interfacial layer when nominally transmitting a polarization component will usually reflect a small fraction of its intensity because of the differing refractive indices of the surrounding prism, the transparent electrodes, and the liquid crystal medium.